1. Field of the Invention
This invention relates generally to testing or burn-in sockets and carriers for semiconductor dice and, more specifically, to an apparatus and method for testing dice which use ball grid array (xe2x80x9cBGAxe2x80x9d) technology, wherein the socket or carrier includes retaining elements, a force system, and a removable die contact insert capable of interfacing with chip scale package (xe2x80x9cCSPxe2x80x9d) BGA dice received within the carrier.
2. State of the Art
Semiconductor dice are used in virtually every electronic device because they are versatile and compact. In fact, each year technology advances, allowing for smaller semiconductor dice and resulting in smaller electronic devices. Although semiconductor dice are functional at the time they are created, inherent manufacturing defects, caused by factors such as contamination or process variability, are generally expected in some percentage of dice. Dice with inherent manufacturing defects have shorter lifetimes than dice without such defects and are the largest contributors to early-life failure rates, or xe2x80x9cinfant mortality.xe2x80x9d Semiconductor manufacturers perform test processes to discover dice with these types of inherent manufacturing defects and achieve a lower early-life failure rate, thereby increasing product reliability.
xe2x80x9cBurn-inxe2x80x9d refers to the process of accelerating early-life failures. This is done by cycling a semiconductor die through a series of stresses at raised temperature designed to simulate extreme field conditions to cause failure of the die and remove those dice which would have otherwise failed during early field use. Typical burn-in begins by placing a semiconductor die package into a socket containing probes or terminals for connecting to all electrical inputs and outputs of the die. Testing includes pre-burn-in and post-burn-in testing as well as burn-in testing. Many sockets in the art can be used for many forms of testing and can be either permanently connected to a testing center, or may act as a carrier which is easily moved and attached to one or more different testing centers for various tests.
One concern in relation to BGA die test sockets is that the semiconductor die be held in the socket securely enough to maintain a valid testing process through sufficient continuous electrical communication between the socket and the die, yet not so securely held that the die or its electrical connections are damaged. Examples of test sockets which hold dice with leads in place can be found in U.S. Pat. No. 5,504,436 (Okutsu, 1996), and U.S. Pat. No. 5,088,930 (Murphy, 1992). However, these sockets only work to hold the die in place if the electrical connections are of specific given types, namely extending leads. Examples of test sockets which hold BGA dice in place can be found in U.S. Pat. No. 5,531,608 (Abe, 1996), and U.S. Pat. No. 5,518,410 (Masami, 1996). However, none of these conventional sockets can adequately test CSP BGA dice because the array of terminals in a CSP BGA die is significantly smaller and of finer pitch (i.e., spacing between ball centers) than larger scale BGA dice.
A second concern, related to the first, is that the test probes used within a socket have sufficient rigidity and conductive capacity to accurately test the die. As semiconductor dice and their conductive elements get smaller, testing of those dice gets more difficult. For example, the test probes used to communicate with the BGA die conductive element array in U.S. Pat. No. 5,518,410 to Masami and U.S. Pat. No. 5,531,608 to Abe, although apparently sufficient for larger scale BGA dice, are not practical for use with CSP BGA dice due to the fine pitch array of minute balls employed. Using known materials and technology to make the probes small enough to distinctly test each conductive element (ball) in the array creates test probes which are insufficiently rigid and/or have insufficient conductive capacity. If test probes are insufficiently rigid, they may bend or break, causing the socket to perform an inaccurate test process. Furthermore, if the test probes have insufficient conductive capacity, they may fail or give inaccurate results. Current technology does not yet permit manufacture of probes small enough to adequately test CSP BGA dice while maintaining the required probe rigidity and conductive capacity.
A third concern in relation to test sockets is minimizing the number of automated operations required to load and unload a socket, yet maintain simplicity of socket design. Many conventional sockets and carriers used for testing non-packaged and non-encapsulated dice include multiple parts or parts which must be disassembled to insert or remove a die from the socket or carrier, thus requiring additional automated steps. An example of a carrier with an assembly which must be disassembled to insert or remove a die is disclosed in co-owned U.S. Pat. No. 5,519,332 to Wood et al. (May 21, 1996), herein incorporated by reference. One advantage of using a carrier which must be disassembled, such as that disclosed by Wood et al., is there are fewer moving parts than in carriers which do not require disassembly for use and, thus, less opportunity for mechanical failure. Carriers and sockets in the current art for testing BGA dice which do not require disassembly to insert and remove dice, although they require fewer automated operations, also contain many moving parts. This presents greater opportunity for malfunction and error.
A fourth concern in relation to test sockets used in automated test processes is to avoid lids or other socket parts which protrude so far they interfere with the automated processes. Many sockets in the art for testing BGA dice include hinged lids which extend well beyond, or above, the socket and thus may be broken off during automated processes. This result is clearly undesired, as it causes delay, causes possible equipment damage, adds expense for repair, and causes lower die yield.
A fifth concern in dealing with BGA dice in test sockets is the build-up of static electricity on the equipment. Current BGA interface die test processes typically include the steps of opening the socket, placing the die within the socket, releasing the die, then closing the socket. Although this may work for larger scale BGA devices which are sufficiently heavy to overcome the static electricity created between the releasing device and the die, it may not work for CSP BGA die test processes. A specific problem experienced more often when testing CSP BGA dice is that static causes the dice to stick to the releasing device instead of remaining in the socket.
It would be advantageous to have a die socket and carrier for use with CSP BGA interface dice which holds the die within the socket, has few moving measuring parts, does not require disassembly to insert and remove a die, has a low profile lid and has terminals adequate to accurately interface with and test a CSP BGA die. Furthermore, it would be advantageous to have a method for testing BGA dice which overcomes the static electricity problem.
According to the present invention, a test socket assembly is disclosed wherein a removable die contact insert, having terminals of sufficient number and disposed to distinctly and accurately interface with and test a CSP BGA die, is disposed within the test socket containing retaining elements and a force system. In general, the invention includes a test socket assembly comprising an electrically insulating base containing a die contact insert and electrically insulating die retaining elements which, in cooperation with the base, apply pressure against the back side of a BGA die to maintain continuous contact between the conductive element array of the die and an array of electrically conductive contacts or terminals on the die contact insert. By operating the retaining elements appropriately, the socket may be opened to insert or remove a BGA die from the socket.
In a particular and preferred aspect of the invention, the die contact insert comprises electric terminals in an array in mirrored orientation to that of the conductive elements of a CSP BGA die and is dimensioned such that each conductive element in the array is discretely connected to the socket in electrical communication sufficient to test the die. In one embodiment, the die contact insert is removable and interchangeable. In this way, versatility is afforded for multiple conductive element array configurations using the same socket. In a more simple embodiment, the die contact insert is affixed to or integral with the base. In another embodiment, the electric terminals are wells having electrically conductive material which extend in conductive paths to a peripheral side of the die contact insert. The paths are then adapted to communicate with a testing station or other external station. Such an adaptation allows for mobility and easier connection to the station. In still another embodiment, the electric terminals of the die contact insert employ conductive paths to communicate with a corresponding external interface integral with the base. Such base external interface can then be fit into an existing socket of a burn-in board or otherwise connected to a testing station.
In another particular and preferred aspect of the invention, a vertical force is used to assist in maintaining sufficient continuous electrical contact between the BGA die terminals and the die contact insert. In one preferred embodiment, the vertical force is applied by the combination of an insulating plate suspended and urged toward a retaining element by at least one spring. The die contact insert is disposed between the insulating plate and the BGA die such that when the socket is closed, vertical force is exerted toward the BGA die, causing it to remain in substantially continuous electrical contact with the die contact insert. In another preferred embodiment, the retaining element comprises retention tongs which move between open and closed positions by applying or releasing pressure on a tong activating frame. In still another embodiment, the upper member comprises a clamshell lid, spring-loaded latch and resilient foam member. When the clamshell lid is moved into the closed position over a die, the resilient foam member applies downward pressure on the die. Yet another embodiment comprises retention tongs, each having an end affixed to at least one spring urging the retention tong toward the base. Thus, a force is applied from the tongs to any die placed within the socket. The socket is opened and closed by applying pressure on a tong activating frame which moves the retention tongs into an appropriate position. In still yet another embodiment, a low-profile lid is pivotally attached to the insert to open or close the socket and is held closed by a latch. Using a lid allows for a portable robust package which may be transferred between various test processes. Using a low-profile lid, the robust package may be used in automated processes more easily and with less risk that the lid will be broken off.
A method for testing CSP BGA dice is also disclosed wherein the static electricity problem experienced in prior art is overcome. According to the method, a BGA die is brought above a vertical compression test socket by a die deposit probe surrounded by a sufficiently rigid sleeve. The sleeve is independently lowered to apply a vertical force to a retaining element activating mechanism, thus opening the socket. The die deposit probe then lowers the die into, and aligned with, the socket containing an insert for accommodating that particular CSP BGA die. Instead of then removing the probe as is currently done in the art, the sleeve is withdrawn to close the socket. With the die held in place, the die deposit probe is then withdrawn. The die is removed by reversing the previous steps.
Other features, advantages, and objects of the present invention will become apparent from a consideration of the drawings and ensuing description.